It is known that natural starch which is found in vegetable products can be treated at elevated temperatures to form a melt.
Such a melt may preferably be formed by heating the starch material above the glass transition and melting temperatures of its components so that such undergo endothermic rearrangement. Preferably the starch material contains a defined amount of a plasticizer, which preferably is water, and melt formation is carried out at an elevated temperature in a closed volume, and hence at an elevated pressure.
It is possible to melt starch substantially in the absence of water, but in the presence of another suitable plasticizer, for example a liquid having a boiling point higher than the starch glass transition and melting temperature.
Different degrees of uniformity in melt formation, which can be measured by various methods, are possible. One method, for example, is to microscopically determine the amount of granular structure remaining in a starch melt. It is preferred that the starch is destructurised, viz, that the melt is substantially uniform in character, that light microscopy at a magnification of about 500.times., indicates a substantial lack of, or reduction in, granular structure, that the starch so melted exhibits little or no birefringence and that X-ray studies indicate a substantial, reduction in, or lack of, starch crystallinity in the melt.
It is implicit in the art of forming thermoplastics that the major components thereof should be of high molecular weight. This is the case also for the use of native starch in melt formation processes and for articles obtained therefrom. However, when blending native starch, in many cases such native starch is difficult to process and also difficult to blend with an alkenol homopolymer and/or an alkenol copolymer, because a relatively high amounts of plasticizer as well as energy input is required in order to achieve uniform melt formation and maximum physical properties of the shaped articles obtained from such a melt. Neat converted starch exhibits enhanced processability but lower strength and elongation to break as expected. The lower molecular weight of converted starch leads to enhanced crystallysability and higher modulus in neat systems.
It has now been discovered that native starch can be replaced by "converted" starches, i.e. a starch with a much lower average molecular weight than native starch. The advantages of using converted starches are enhanced processability of the converted starch/polymer blends. Concomitant the converted starch reduces the amount of plasticizer as well as the energy input necessary to provide uniform melt formation. As a consequence higher production speeds are possible. Surprisingly an improved mixing behaviour in blends with other synthetic polymers is observed resulting in a very uniform and often single phase product. The improved physical properties in blends are novel and, in view of the reduced average molecular weight of the starch, also very surprising.
Converted starches are prepared by degradation of starch molecules yielding products of lower dispersion viscosities than the original starch. Such products are known. Although many of the properties of the original starch are changed during the conversion process the main purpose of said process is to reduce the viscosity of the raw starch. The conversion process involves breaking, rearranging and/or recombining the starch chains for example in the presence and through the action of acids, alkalies, enzymes, oxidizing agents and/or heat. An important effect is the cutting of the chain lengths to lower average molecular weights. Controlled acid hydrolysis yields "thin-boiling" or "fluidity" starches in a wide range of viscosities wherein this hydrolysis is carried out below the gelatinization point of the starch. Acid hydrolysed corn starch is most preferred. The higher the "fluidity" the more degraded is the starch and in consequence the less, viscous is the dispersion for a given concentration. Acid conversion is preferred due to the ease of handling and recovery afforded by a granular starch as for example opposed to starch in dispersed form as necessitated by enzyme conversion. However, the means of producing the converted starch is of no importance for carrying out the present invention. These starches are generally named "converted starch" and this term will be used herein.
The degree of conversion is given herein as a 8.8% solids calcium chloride viscosity in seconds. Such calcium chloride viscosity values are known in the art and are for example described in the U.S. Pat. No. 4,726,957, which procedure is especially suitable for high amylose starch. The procedure of U.S. Pat. No. 4,726,957 can, for example, be modified slightly in that the weight of anhydrous starch is 9.0 grams and 125 grams of 40% calcium chloride solution is used.
U.S. Pat. No. 4,207,355 describes a water fluidity test which is most suitable for all other starches, i.e. starches that are not classified as high amylose starch or contain less than about 30% amylose. In carrying out the present invention it is recommended that the procedure of U.S. Pat. No. 4,726,957 is used for high amylose starches and the procedure of U.S. Pat. No. 4,207,355, is used for all other starches. Of course it is possible to further modifiy these tests. It is no problem for the person skilled in the art to correlate the different viscosity values obtained by the different modified test procedures.